1. Field of the Invention
The present invention relates to hearing aids. The invention, more specifically, relates to analog-to-digital input signal converters in digital hearing aids. The invention further relates to a method of converting an analog signal to a digital signal in a hearing aid.
An analog-to-digital converter, denoted an A/D converter in the following, converts a varying current or voltage into a digital data format. Several different A/D converter topologies exist, each having benefits and tradeoffs in terms of conversion speed, accuracy, quantization noise, current consumption, word length, linearity and circuit complexity. In contemporary, digital hearing aid designs, the delta-sigma A/D converter type is the preferred converter type due to a number of important factors such as easy aliasing filter implementation, conversion noise being controllable by design, comparatively low power consumption and relatively easy implementation due to a low component count when compared to existing A/D converter designs.
By definition, noise inherent in a signal processing device is unwanted signals introduced by the signal processing device itself. Inherent noise may e.g. originate from inadequate operating conditions, poor design or variations in component values. These circumstances have to be taken into account in designing the signal processing device. In A/D converters, several different types of noise may be observed. Among these are conversion noise, quantization noise, thermal noise, flicker noise, recombination noise, and noise due to various physical limitations in the gain-producing elements. In order to provide a distinction between the sources of these different noise types, the most important noise types will be discussed briefly in the following.
Quantization noise originates from the process of quantifying a continuous input voltage span into a finite set of voltage levels that may be represented by discrete, binary levels according to the expression:LN=2′where LN is the number of discrete levels possible and n is the number of bits used to represent a single sample in the digital domain. Quantization noise may be thought of as the difference between the actual input voltage of a single sample and the discrete voltage used to represent it. This type of noise may thus be minimized, e.g. by increasing the number of bits representing the signal arbitrarily, and will therefore not be discussed further here.
Thermal noise originates from the random Brownian motion of electrons in a resistive medium. Given a resistance, a bandwidth and a temperature, the rms thermal noise Vnt is given by:Vnt=√{square root over (4kbTΔfR)}where kb is Boltzmann's constant, 1,38065*10−23 J/K, T the absolute temperature in K, Δf the bandwidth of interest in Hz and R the resistance in Ω of the circuit element considered.
Flicker noise, or 1/f noise, is predominant in the noise spectrum at low frequencies. It has been observed in electronic devices since the era of vacuum tubes, and is also present in contemporary semiconductor devices.
Shot noise is the result of stochastic phenomena caused by an electric current crossing a potential barrier, such as the barrier found between P-doped and N-doped material in a semiconductor element. Current shot noise In is a temperature-independent quantity, and may be described by the expression:In=√{square root over (2qI)}where q is the electron charge, 1,602*10−19 Coulombs, and I the bias current for the semiconductor element. The unit of the spectral density of shot noise is A/√{square root over (Hz)}.
In order to provide a hearing aid capable of working uninterrupted for several days without a need for replacing the battery, one design goal for the hearing aid is that the current drawn from the battery by the electronic circuit is reduced as much as possible, preferably to a value below 1 mA. A semiconductor element providing amplification in the order of between one hundred times to perhaps a thousand times the signal present at its input uses a considerable percentage of this current as its bias current in order to handle the large gain within its operating limits. From the foregoing it is evident that shot noise is dependent on the current flowing through the semiconductor element, this fact providing further motivation for reducing the bias current for the amplifier in the A/D converter as much as possible.
2. The Prior Art
Delta-sigma A/D converters are well known in the art. Their purpose is to convert a varying, analog input voltage into a binary bit stream for further processing in the digital domain. Delta-sigma A/D converters have significant advantages over other A/D converter designs. They have a relatively low component count, and they feature various signal processing advantages above other A/D converter designs. In order to reduce conversion noise, oversampling is used. By measuring each discrete voltage many times, e.g. 64, errors due to statistical variations in the input signal are leveled out, and the conversion noise spectrum is pushed far beyond the Nyquist limit, thus making conversion noise very easy to filter out from the signal. One drawback is that the converter clock rate in this example has to be 64 times the desired sample clock rate.
In its essence, a delta-sigma A/D converter comprises a delta-sigma modulator and a low-pass filter. This may be made with an integrator, a comparator and a D-flip-flop. The output signal of the flip-flop is fed back through a feedback loop comprising a one-bit D/A converter, and is subtracted from the input signal upstream of the integrator. The subtracted feedback signal provides an error signal to the input of the delta-sigma modulator.
The error signal from the feedback loop of the A/D converter is used to ensure that, on average, the output signal level of the converter is always equal to the input signal level. If no signal is present on the converter input, a symmetric output bit stream of binary ones and zeroes is generated by the A/D converter. When the input signal voltage changes to a more positive voltage, more binary ones will be present in the output bit stream, and when the input signal voltage changes to a more negative voltage, more binary zeroes will be present in the output bit stream. The delta-sigma A/D converter thus converts an analog input signal into a balance between ones and zeroes in the output bit stream.